Rechargeable batteries, most notably nickel-cadmium (NiCad) batteries, are increasingly being used for heavy applications such as portable power tools. They have enjoyed a rather long history as a standard power cell for moderate to heavy loads, as in power supplies for remote radio equipment in police, fire, emergency medical, and other communications, particularly vehicular communications. In all of these cases, rapid charging capability is desirable. The technology of NiCad batteries has progressed quite well, so that their electrochemistry and physical properties are capable of supporting rapid charging.
The conventional recommended method for charging NiCad batteries has been the "ten hour" cycle; a cell or battery that is rated to give up its full power at the maximum recommended drain rate over a ten hour period is charged at that same current but over a fourteen hour period. The additional charge time is expected to make up for charging inefficiencies. Ostensibly, this cycle matches well the eight hour shifts of users who might employ these batteries in radio communications. Unfortunately, this rhythm is a virtual fantasy. Portable radio transceivers produce a quite limited drain while receiving, especially in a squelched stand-by mode, but constitute heavy loads during transmission. The so-called "ten hour" cycle usually overcharges or undercharges the NiCad battery, and usually does not discharge the battery to near its exhausted state, so that fourteen hours of unattended charging damages the battery. Also, mobile or portable radio equipment is not usually in excess availability at its typical sites of usage; successive work shifts inherit the equipment with the batteries in an unknown state of charge or utilize replacement batteries that have also been subjected to an unknown service cycle.
A fully discharged battery usually produces a very low output voltage. Although its voltage increases rapidly at the onset of charging, due to its high internal impedance when discharged, and the availability of charging current from the charger across that impedance, nonetheless the total excursion of the battery voltage, even ignoring the rapid rise in the first moments, can be several volts. The actual rate of change of the voltage, ignoring the first moments in the case of a fully discharged battery, is nonetheless fairly small, even for high charging rates. The battery acts to "swamp out" radical changes, acting somewhat as a voltage "buffer". The ideal voltage of the chemical system of the battery is quite fixed; the small changes seen during charging are largely an effect of internal impedance changes.
The total excursion of a battery's voltage during charging, even ignoring the initial rapid slewing of voltage when charging a totally discharged battery, can be substantial; it can be several volts for a multi-cell battery. But the actual absolute voltage is a poor indicator of charge, considering things such as age and previous duty cycle. As an example, consider a hypothetical twenty-four volt battery in a state of absolute discharge. A voltmeter placed across it reads essentially zero volts.
When charging starts, the battery voltage rises within seconds to some fairly high voltage, say nineteen volts. This might be the case even if less than a ten-thousandth of its capacity had been restored in the initial seconds of charging. Over a succeeding period, presuming a rapid charging cycle, the battery voltage rises, at a fairly steady rate, from nineteen volts to some final peak voltage, somewhere between twenty-two and twenty-six volts. The specific peak voltage is determined by a number of factors, such as the specific chemistry of the battery, its age, its designed current delivery capacity, and the nuances of internal impedance that are influenced by prior usage cycles. Therefore, measurement of its output voltage is not a good clue as to its state of charge.
Even using a controlled rate of charge (a constant, controlled current, no matter the required voltage), there is no way to be sure that a given amount of charging time is adequate or excessive to charge the battery fully, since it cannot be known what the original degree of discharge was, nor the degree of efficiency of charging, nor for that matter the particular full charge capacity (consider both low and high capacity batteries of the same voltage, and the differences in capacities of batteries of even the same nominal capacity). Neither time nor voltage is a good indicator of charging completion.
Moreover, in typical usage (e.g., a power tool) a heavy duty cycle battery is exhausted rapidly, sometimes in substantially less than an hour, and then rapidly charged, often in an even shorter time. The extreme charging currents employed can be catastrophic to the battery (and hazardous to personnel, the charging equipment, and other local equipment) unless there is a good method for assessing the battery's state of charge, with concomitant careful management of the charging cycle.
There are a number of other strategies for charging NiCad batteries. An old "standby" is the trickle charge. This regime uses a voltage equal to or very slightly higher than the fully charged voltage of the ideal battery. The available current is limited so that even if the battery is almost fully discharged, the initial current is not excessive. This limited current charges the battery quite slowly. As the battery voltage rises, there is a higher back e.m.f. so that the charging current decreases. Theoretically, at full charge the current is so reduced as to be unable to damage the battery by overheating. By this time, also, the voltage differential is theoretically small enough to be well below the required hydrolysis voltage of the electrolyte (typically an aqueous solution), so that gas due to hydrolysis should not form. The problems of this trickle system are many. Two most serious are that: (1) the rate of charge is dismally low, rendering this method suitable only for low-current standby systems; and (2) as batteries age, their terminal voltage changes. Even if great care is taken to trim the maximum available charging voltage to the original optimum level, this becomes incorrect for older batteries.
A related method is voltage-tapered charging, which is similar to trickle charging except that less impedance is used to limit the initial current. This provides a faster early charge than trickle charging. In a similar way, the differential between the maximum available charging voltage and the battery's own voltage is depended upon to reduce the current automatically as the battery proceeds to charge. Because of the lower circuit impedance, the user is relied upon to terminate the charge cycle. The effect of aging battery terminal voltage is more pronounced in this type of charger, so that overcharge damage can be severe without the user's intervention. Many so-called "ten hour" (fourteen hour charge) systems actually rely on this method.
Other systems charge a battery in an essentially non-tapered manner, and depend upon sensing of the battery voltage or sensing of a rise in battery temperature to signal the end of a charge cycle. For old batteries, or those that have gone through irregular charge/discarge cycles, the voltage at the battery terminals at full charge can be subject to a substantial shift from that which is expected, making the simple voltage-sensing method unreliable and sometimes quite dangerous. The thermal cut-off approach can be relatively reliable in carefully controlled environments (where random external cooling is not a factor), but is less than effectual because it usually relies upon the battery becoming undesirably overheated before cut-off is performed, thereby actually promoting a diminution of battery service life and power delivery ability. In the case of modern extremely rapid charge cycles, the current is so high that a battery can be seriously damaged before the build-up of internal heat can be reliably sensed and used to stop the charge cycle.
One property of secondary cells, not exclusive to the NiCad systems, can be, and has been exploited to get around these difficulties. This involves the behavior of these electrochemical cells as they approach and begin to pass the peak charge point. A minute drop in terminal voltage occurs, rather than a continual increase. The voltage drop is due to a combination of effects, ostensibly electrode polarization and some thermal effects. This occurs quite a bit earlier than a rise in external temperature, and before cell damage has occurred. The key is to sense this change in a reliable, versatile, and inexpensive manner. Factors affecting reliability are those that rest upon making this determination in spite of battery characteristics such as age or prior duty cycle, and without regard to errors caused by circuitry calibration, drift, or leakage. Factors affecting versatility concern the ability to perform this feat for widely differing cell types and chemistries and even greatly differing battery voltages (e.g., the ability to charge a 6 volt battery with the same equipment as a 9 or 12 volt battery).
Mobile battery packs are routinely used. The acts performed range from simple communications and local vital signs monitoring, to bio-telemetry of this information, and certain critical applications such as ventricle defibrillators for delivery of rather high powered, but carefully regulated dosages of electrical shock for cardiopulmonary resuscitation. In specialized cases, remote x-ray, bio-chemical analysis by electronic instrumentation, and even medical ultrasound examination is performed. To insure the fitness of the battery system, advanced measures are not spared, and microprocessor based systems, often already present within the equipment, are employed to monitor and control the charge of the battery pack. This is performed by frequently and periodically converting the measured battery voltage to a digital value that is stored and compared to subsequent measurements of that same quantity during charge.
If intelligently programmed, such a system can be "smart" enough to ignore treacherous variables such as battery age, specific chemistry, prior history, and even specific battery voltage, in order to tailor an appropriate and optimal charge cycle. In an expensive instrument, it is practical to interface electronics to extract from existing components the functionality that would otherwise cost too much to implement in a microprocessor-based stand-alone battery charger. Unfortunately, this option is not available for the user of a cordless drill or walkie-talkie.
Somewhat less expensive alternatives of this "slope-delta inflection" method have also been effected. A ramp-generator-cum-feedback with comparator and logic circuitry solution has been proposed. In this case, essentially no attention was paid to the case of improper use, such as an attempt to charge an already-charged battery, or the peculiarities of the response of older or mis-cycled cells. The actual circuitry was also complex and ill-suited to generalization. Other methods are conceivable, such as the use of sample-and-hold circuitry. These usually require a circuit topology that is highly dependent upon the leakage of a long-time-constant RC network, especially since the period of measurement extends over many seconds of time, and must be responsive to a few tens of millivolts versus a battery voltage that may range to several tens of volts. In addition, the necessity to perform the sample-and-hold function spawns an entire order of complexity that can be shown to be avoidable.